1. Field of the Invention
The present invention relates to a locking apparatus for an optical frequency multiplex system, and more particularly to a locking apparatus for an optical frequency multiplex system capable of applying low frequency dithering signals to individual light sources, detecting an error signal for locking at the minimum value point of reflection outputs of an optical fiber filter used as a resonator, thereby enabling a reduction in the number of error signals per channel and accommodating of an increased number of channels.
2. Description of the Prior Art
Generally, an optical multiplexor is a device for optically transferring, through a single optical fiber, several basic transfer signals grouped together The amount of information being transferred is determined by the product of the transfer rate of the basic transfer signals multiplied by the number of groupable signals, namely, the number of channels.
However, a recent progressive increase in demand for broad-band services such as multimedia has resulted in excess information to be transferred over the usable transfer capacity in essential transfer sections. As a result, there is a demand for a new multiplex system, such as an optical frequency multiplex system.
The optical frequency multiplex system is a system constructed, in a transmitting side thereof, to receive modulated optical signals respectively outputted from several light sources; align the received optical signals respectively with predetermined optical frequency slots; group the aligned signals; and then transmit the grouped signals. A receiving side of the system separates and extracts respective optical signals aligned in the frequency slots by the use of optical filters to reproduce the signals.
A commonly used method for such an optical frequency multiplex system for transferring a large amount of information is to maximize the transfer rate per channel within a limited range allowed by currently existing techniques, and then increasing the number of multiplex channels, as compared to subscriber channels. However, such an optical frequency multiplex system is dependent on the system used for generating the frequency slots required for the multiplex system.
In order to solve the above-mentioned problem, there has recently been proposed the use of an optical fiber filter as a resonator and the use of resonance frequencies as slots for the multiplex system.
Referring to FIG. 1, there is illustrated a conventional locking apparatus for an optical frequency multiplex system. As shown in FIG. 1, the locking apparatus includes a transmitting unit 1 comprised of a plurality of light sources 2. Light emitted by the light sources 2 is coupled together by a photocoupler 4. The coupled light outputted from the photocoupler 4 is then sent to an optical fiber filter 5 which is used as a resonator. The light is transmitted through successive resonance frequency components of the optical fiber filter 5. The optical output from the optical fiber filter 5 has a maximum level when the light propagating in the optical fiber filter 5 corresponds to the resonance frequency of the optical fiber filter 5. When the light transmitting the optical fiber filter 5 does not correspond to the resonance frequency of the optical fiber filter 5, the level of the optical output from the optical fiber filter 5 is decreased. Accordingly, the center frequency (or center carrier) is adjusted using an optical frequency stabilizing controller 7 so as to always obtain the maximum optical output detected by a photodetector 6. By this adjustment, optical frequencies of individual light sources 2 correspond to successive resonance frequencies of the optical fiber filter 5, respectively.
When multichannel frequencies are stabilized using the transmitted light beams, however, the intensity of the transmitted light is increased according to the number of multiplexing channels. As a result, the photodetector receiving the multiplexed transmitting light may be saturated when it receives more than a certain amount of transmitted light. In such a case, the signal discriminating ability of the photodetector may be degraded. As a result, it is impossible to obtain any feedback signal.